1. Field of the Invention
This invention relates to a system for rehabilitation of a hearing disorder which comprises at least one sensor which picks up sound, an electronic signal processing unit for audio signal processing and amplification, an electrical power supply unit which supplies individual components of the system with current, and one or more electroacoustic, electromechanical or purely electrical output-side actuators or any combination of these actuators for stimulation of damaged hearing.
2. Description of the Related Art
Hearing systems are defined here as systems in which the acoustic signal is picked up with at least one sensor which converts the acoustic signal into an electrical signal (microphone function), which is electronically further processed and amplified, and whose output-side signal stimulates the damaged hearing acoustically, mechanically or electrically or by some combination of these three forms of physical stimulation.
The expression “hearing disorder” is defined here as all types of inner ear or retro-cochlear damage, combined inner ear and middle ear damage, and also temporary or permanent noise impression (tinnitus).
Conventional hearing aids with output-side acoustic stimulation of damaged hearing, especially of the inner ear, in recent years have undergone major improvements with respect to electronic signal processing which are based especially on use of modern fully digital signal processors. Using these processors and the corresponding signal processing software, the rehabilitation of a hearing disorder can be optimized by refined matching to the individual hearing damage. In particular, for the first time, noise or interfering signal-suppressing algorithms can be implemented which especially take into account the circumstance that mainly those with sensorineural hearing damage have major problems in understanding of speech of individuals when they are in a noisy environment.
In recent years rehabilitation of sensorineural hearing disorders with partially implantable electronic systems has acquired major importance. In particular, this applies to the group of patients in which hearing has completely failed due to accident, illness or other effects or in which hearing is congenitally non-functional. If, in these cases, only the inner ear (cochlea), and not the neural auditory path which leads to the brain, is affected, the remaining auditory nerve can be stimulated with electrical stimulation signals. Thus, a hearing impression can be produced which can lead to speech comprehension. In these so-called cochlear implants (CI), an array of stimulation electrodes, which is controlled by an electronic system (electronic module) is inserted into the cochlea. This electronic module is encapsulated with a hermetical, biocompatible seal and is surgically embedded in the bony area behind the ear (mastoid). The electronic system contains essentially only decoder and driver circuits for the stimulation electrodes. Acoustic sound reception, conversion of this acoustic signal into electrical signals and their further processing, always takes place externally in a so-called speech processor which is worn outside on the body. The speech processor converts the preprocessed signals into a high frequency carrier signal which, via inductive coupling, is transmitted through the closed skin (transcutaneously) to the implant. The sound-receiving microphone is always located outside of the body, and in most applications, in a housing of a behind-the-ear hearing aid worn on the external ear. The microphone is connected to the speech processor by a cable. Such cochlear implant systems, their components, and the principles of transcutaneous signal transmission are described, by way of example, in published European Patent Application EP 0 200 321 A2 and in U.S. Pat. Nos. 5,070,535, 4,441,210, 5,626,629, 5,545,219, 5,578,084, 5,800,475, 5,957,958 and 6,038,484. Processes of speech processing and coding in cochlear implants are described, for example, in published European Patent Application EP 0 823 188 A1, in European Patent 0 190 836 A1 and in U.S. Pat. Nos. 5,597,380, 5,271,397, 5,095,904, 5,601,617 and 5,603,726.
In addition to rehabilitation of congenitally deaf persons and those who have lost their hearing using cochlear implants, for some time, there have been approaches to offer better rehabilitation than with conventional hearing aids to patients with a sensorineural hearing disorder which cannot be surgically corrected by using partially or totally implantable hearing aids. The principle arises, in most embodiments, in stimulating an ossicle of the middle ear or directly stimulating the inner ear via mechanical or hydromechanical means, and not via the amplified acoustic signal of a conventional hearing aid in which the amplified acoustic signal is supplied to the external auditory canal. The actuator stimulus of these electromechanical systems is accomplished with different physical transducer principles, such as, for example, by electromagnetic and piezoelectric systems. The advantage of these devices is seen mainly in the sound quality which is improved as compared to conventional hearing aids, and for totally implanted systems, in the fact that the hearing prosthesis is not visible.
Such partially and fully implantable electromechanical hearing aids are described, for example, by Yanigahara et al. “Implantable Hearing Aid”, Arch Otolaryngol Head Neck Surg-Vol 113, 1987, pp. 869-872; Suzuki et al. “Implantation of Partially Implantable Middle Ear Implant and the Indication”, Advances in Audiology, Vol. 4, 160-166, Karger Basel, 1988; H. P. Zenner et al. “First implantations of a totally implantable electronic hearing system for sensorineural hearing loss”, in HNO Vol. 46, 1998, pp. 844-852; H. Leysieffer et al. “A totally implantable hearing device for the treatment of sensorineural hearing loss: TICA LZ 3001”, HNO Vol. 46, 1998, pp. 853-863; H. P. Zenner et al. “Active electronic hearing implants for patients with conductive and sensorineural hearing loss—a new era of ear surgery”, HNO 45: 749-774; H. P. Zenner et al. “Totally implantable hearing device for sensorineural hearing loss”, The Lancet Vol. 352, No. 9142, page 1751; and are described in numerous patent documents among others in published European Patent Applications EP 0 263 254 A1, EP 0 400 630 A1, and EP 0 499 940 A1, and in U.S. Pat. Nos. 3,557,775, 3,712,962, 3,764,748, 5,411,467, 4,352,960, 4,988,333, 5,015,224, 5,015,225, 5,360,388, 5,772,575, 5,814,095, 5,951,601, 5,977,689 and 5,984,859. Here, the insertion of an electromechanical transducer through an opening in the promontory for direct fluid stimulation in the inner ear is described in U.S. Pat. Nos. 5,772,575, 5,951,601, 5,977,689 and 5,984,859.
Recently, partially and fully implantable hearing systems for rehabilitation of inner ear damage have been in clinical use. Depending on the physical principle of the output-side electromechanical converter, and especially its coupling type to the ossicle of the middle ear, it happens that the attained results of improving speech understanding can be very different. In addition, for many patients, a sufficient loudness level cannot be reached. This aspect is spectrally very diverse; this can mean that, at medium and high frequencies, for example, the generated loudness is sufficient, but not at low frequencies, or vice versa.
Furthermore the spectral bandwidth which can be transmitted can be limited, thus, for example, to low and medium frequencies for electromagnetic converters and to medium and high frequencies for piezoelectric converters. In addition, nonlinear distortions, which are especially pronounced in electromagnetic converters, can have an adverse effect on the resulting sound quality. The lack of loudness leads especially to the fact that the audiological indication range for implantation of an electromechanical hearing system is very limited. This means that patients, for example, with sensorineural hearing loss of greater than 50 dB HL (hearing loss) in the low tone range can only be inadequately supplied with a piezoelectric system. Conversely, pronounced high tone losses can only be poorly supplied with electromagnetic converters.
Many patients with inner ear damage also suffer from temporary or permanent noise impressions (tinnitus) which cannot be surgically corrected and for which, to date, there are no approved drug treatments. Therefore, so-called tinnitus maskers (International Patent Application Publication WO-A 90/07251, published European Patent Application EP 0 537 385 A1, German Utility Model No. 296 16 956) are known. These devices are small, battery-driven devices which are worn like a hearing aid behind or in the ear and which, by means of artificial sounds which are emitted into the auditory canal, for example, via a hearing aid speaker, psychoacoustically mask the tinnitus, and thus, reduce the disturbing noise impression, if possible, to below the threshold of perception. The artificial sounds are often narrowband noise (for example, third-band noise). The spectral position and the loudness level of the noise can be adjusted via a programming device to enable adaptation to the individual tinnitus situation as optimally as possible. In addition, the so-called retraining method has been developed recently in which, by combination of a mental training program and presentation of broadband sound (noise) near the auditory threshold, the perceptibility of the tinnitus in quiet conditions is likewise supposed to be largely suppressed (H. Knoer “Tinnitus retraining therapy and hearing acoustics” journal “Hoerakustik” February 1997, pages 26 and 27). These devices are also called “noisers”.
In the two aforementioned methods for hardware treatment of tinnitus, hearing aid-like, technical devices must be carried visibly outside on the body in the area of the ear; these devices stigmatize the wearer and, therefore, are not willingly worn.
U.S. Pat. No. 5,795,287 describes an implantable tinnitus masker with direct drive of the middle ear, for example, via an electromechanical transducer coupled to the ossicular chain. This directly coupled transducer can preferably be a so-called “Floating Mass Transducer” (FMT). This FMT corresponds to the transducer for implantable hearing aids which is described in U.S. Pat. No. 5,624,376.
In commonly owned, co-pending U.S. patent application Ser. Nos. 09/372,172 and 09/468,860, which are hereby incorporated by reference, implantable systems for treatment of tinnitus by masking and/or noiser functions are described, in which the signal-processing electronic path of a partially or totally implantable hearing system is supplemented by corresponding electronic modules, such that the signals necessary for tinnitus masking or noiser functions can be fed into the signal processing path of the hearing aid function and the pertinent signal parameters can be individually adapted by further electronic measures to the pathological requirements. This adaptability can be accomplished by storing or programming the necessary setting data of the signal generation and feed electronics by using hardware and software in the same physical and logic data storage area of the implant system, and by controlling the feed of the masker or noiser signal into the audio path of the hearing implant via the corresponding electronic actuators.
The above described at least partially implantable hearing systems for rehabilitation of inner ear damage, which are based on an output-side electromechanical transducer, differ from conventional hearing aids essentially only in that the output-side acoustic stimulus (i.e., an amplified acoustic signal in front of the eardrum) is replaced by an amplified mechanical stimulus of the middle ear or inner ear. The acoustic stimulus of a conventional hearing aid ultimately leads to vibratory, i.e., mechanical, stimulation of the inner ear, via mechanical stimulation of the eardrum and the subsequent middle ear. The requirements for effective audio signal preprocessing are fundamentally similar or the same. Furthermore, in both embodiments on the output side a localized vibratory stimulus is ultimately routed to the damaged inner ear (for example, an amplified mechanical vibration of the stapes in the oval window of the inner ear).
Basically, in this routinely used rehabilitation of inner ear damage by active hearing systems (regardless of whether the rehabilitation is by an external acoustic stimulus or by an implanted electromechanical stimulus), at present, there is a major drawback which is described below in summary for understanding of this invention: most cases of sensorineural deafness are based on more or less pronounced damage of the outer hair cells in the inner ear. These outer hair cells, which in large number are located in the organ of Corti along the basilar membrane, form part of the so-called cochlear amplifier which, depending on local excitation of the basilar membrane as a result of traveling wave formation, actively mechanically de-attenuate this local stimulation range at low levels, and thus, small traveling wave amplitudes, which leads to an increase in sensitivity. This active amplification is based on a very complex, efferently controlled process which is not described here. Furthermore, it is assumed that, at very high levels of inner ear stimulation as a result of the high loudness, this effect is reversed in its action, and thus, locally reduces and actively attenuates the traveling wave amplitude. These nonlinear characteristics of the cochlear amplifier, which is located along the organ or Corti in several hundred functional units with locally limited action, are of decisive importance for the function of the healthy inner ear. In partial or total failure of the outer hair cells, in addition to a loss of sensitivity, which leads to a rise in the hearing threshold, other disadvantages arise: the described active de-attenuation of the basilar membrane leads to high Q-factors of the envelopes of the traveling waves (tuning curves) which are essentially responsible for the frequency differentiation capacity (tone pitch differences). If this high quality is lacking due to failure or partial damage of the outer hair cells, the affected individual can perceive tone pitch differences much more poorly. The rise of the hearing threshold leads, moreover, to a reduction of the dynamic range since the upper sensory boundary (discomfort threshold) in labyrinthine deafness does not rise at the same time. This reduction of dynamics results in an increased perception of loudness which is called positive recruitment. The described effects, which are caused by damage or failure of the outer hair cells, lead, in the overall effect for the affected individual, to a reduction in speech comprehension, especially in a noisy environment (summary description in Zenner, H. P.: Hearing, Georg Thieme Verlag Stuttgart, New York, 1994, pages 20-23, 107 and 108, and LePage, E. W., Johnstone, M. B.: “Non-linear mechanical behavior of the basilar membrane in the basal turn of the guinea pig cochlea.” Hearing Research 2 (1980), pp. 183-189).
An important consequence of this described mechanism is that, as indicated above, both in conventional acoustic hearing aids and also in partially or fully implantable hearing systems, the important functions of the damaged outer hair cells, and thus, of the cochlear amplifier, cannot be replaced or at least partially restored. U.S. Pat. No. 6,123,660 discloses a converter arrangement for partially or fully implantable hearing aids for direct mechanical excitation of the middle ear or inner ear, which is provided with a piezoelectric converter element and also with an electromagnetic converter which are accommodated in a common housing and the two can be coupled via the same coupling element to the middle ear ossicle or directly to the inner ear. Furthermore, implantable hearing systems are known (U.S. Pat. Nos. 5,997,466 and 6,005,955) which work with two or more output-side electromechanical converters in one or locally separate arrangements. These embodiments are, however, uniquely described in that a system design with more than one converter enables a linear superposition of the deflection frequency responses of the individual converters which, as a result, allows an output-side excitation form of the cochlea which is adjustable or programmable depending specifically on frequency or spectrally optimized as much as possible and thus will lead to a spectrally balanced and sufficient loudness impression of the implant system. Rehabilitation of the cochlear amplifier with the aforementioned features is, however, not possible by these embodiments or described signal preprocessing methods.
In cochlear implants (CI), solely electrical stimulation signals are now used as the actuator stimuli. After implantation of a CI in completely deaf patients, training is generally necessary for rehabilitation of hearing, since the artificial stimuli must be learned, because the artificial stimuli do not fundamentally correspond to thee biologically proper form of stimulation of the inner ear. Conversely, this rehabilitation phase is omitted after implantation of an electromechanical hearing system in those with hearing difficulties since the mechanical form of stimulation is biologically suitable, as described above, and since the mechanical form of stimulation ultimately largely corresponds with a hearing aid at least with respect to the basic function, i.e., the stimulation at the input of the inner ear is of a vibratory nature.
Recently, it has become scientifically known from CI implantations that even for incomplete deafness cochlear implants (CIs) can be successfully used when sufficient speech discrimination can no longer be achieved with a conventional hearing aid. Interestingly, it was demonstrated that the important inner ear structures which enable residual acoustic hearing capacity can be maintained in part or largely stably over time when a CI electrode is inserted into the cochlea (S. Ruh et al.: “Cochlear implant for patients with residual hearing”, Laryngo-Rhino-Otol. 76 (1997) pp. 347-350; J. Mueller-Deile et al.: “Cochlear implant supply for non-deaf patients?” Laryngo-Rhino-Otol. 77 (1998) pp. 136-143; E. Lehnhardt: “Intracochlear placement of cochlear implant electrodes in soft surgery technique”, HNO 41 (1993), pp. 356-359). In the foreseeable future, it certainly will be possible, in case of residual hearing capacity, to clinically place CI electrodes intracochlearly in a manner such that the remaining inner ear structures can be preserved over the long term, and thus, can continue to be stimulated in a biologically proper manner, i.e., vibrationally, and lead to a usable hearing impression.
Therefore, the recent patent literature contains new approaches to “replacing” the damaged cochlear amplifier by a multichannel intracochlear converter array with a purely mechanical or mixed mechanical/electrical form of stimulation (commonly owned co-pending U.S. patent application Ser. Nos. 09/833,704, 09/833,642, 09/833,643). Whether these solutions will lead to a clear improvement of speech understanding especially in a noisy environment however cannot yet be foreseen.
Therefore, in all the described processes in currently available systems, there remains the serious defect that especially speech understanding in a noisy environment is greatly reduced. This applies especially in cochlear implants in which, due to a physiologically improper form of electrical stimulation, the patients must more or less learn a new language, the understanding or interpretation of which is of course more prone to interfering signal portions.
Especially in fully implantable hearing systems can this defect be reduced by an optimized microphone position near the eardrum (see published European Patent Applications EP 0 831 673 A2, EP 0 831 674 A1 and U.S. Pat. No. 5,814,095) since, in this way, the natural directional action of the outer ear is used. These directional effects are approached in conventional hearing aids by the suitable wiring of several, locally separate microphones. But these measures are relatively ineffectual when the useful and interfering sound comes from the same direction.
This serious defect can be reduced only to a certain extent by modern digital speech process algorithms with interfering signal reduction. For example, the input signal is divided into several frequency bands. In each band, using simple signal-statistical criteria (for example modulation parameters) it is evaluated whether it is a speech or an interference signal in this band. Bands in which the algorithm detects speech signals are raised in their amplification, conversely bands in which mainly interfering sound predominates are attenuated. It is important here that this process which is frequently used today cannot change the signal-noise ratio in a band and thus cannot improve it. It is common to all the analysis processes used that they use only elementary signal-statistical parameters (for example, amplitude modulation depth and frequency).
Further, it is common to all the described rehabilitation processes that they do analyze, process and amplify the audio signal or speech signal, and in cochlear implants, code it into a new form of stimulation. Transmission of direct speech information in more or less real time is however preserved. This results in that interfering signal portions can also be approximately recognized by analysis processes and reduced. Genuine separation of speech and noise information however does not occur.